The present invention relates to equipment for the emission and distribution of light by optical fibres, particularly for in-line spectrophotometric control with the aid of a double beam spectrophotometer.
More specifically, the invention relates to an apparatus making it possible to e.g. determine the content of a certain number of substances within a solution by remote spectrophotometric analysis, the samples to be analysed not being located in the spectrophotometer sample compartment. This is for example the case when the solution to be analysed is radioactive and the cell containing the solution has to be placed within a protective enclosure, or in the remote control of industrial processes.
Among the methods for measuring chemical species in solutions or gases, spectrophotometric analysis is widely used in laboratories. In accordance with Beer's Law, it consists of measuring the light absorption linked, at a predetermined wavelength, with the concentration of the species to be dosed.
In the factory, in areas with a difficult access and more particularly in nuclear chemistry installations, this analysis method has become theoretically usable on an in-line basis, without transfer of samples to the laboratory, through optical fibres coming on to the market which make it possible to transfer information in the form of light from a measuring cell to a spectrophotometer, which can be at a varying distance from the measuring cell or cells, as a function of the circumstances.
The use of optical fibres connecting the measuring cell or probe, positioned in situ in a photometric analysis process consequently represents an advance leading to rendering commonplace equipment considered to be fragile in a corrosive industrial environment for reasons of chemical corrosion, deflagration or radioactivity.
To put this method based on optical fibres into effect, it is possible either to use photometers specially designed for remote measurement purposes, such as those marketed by the firm Hermann-Motriz under the registered trademarks Telephot and Crudmeter, or to equip commercially available spectrophotometers widely used in laboratories with an additional device, such as that described in European Pat. No. 0 015 170 granted on 6/22/83.
Such a known device is described diagrammatically with reference to FIG. 1, which shows a spectrophotometer with a sample compartment S having two beams, respectively designated R for the reference beam or channel and M for the measuring beam or channel. The admissions of light of intensity Io from the monochromator takes place at 2 and 3 in the insertion compartment 4 of the optical coupler. A system 5 of mirrors at 45.degree. makes it possible to transmit this light both towards the optical reference probe 6 and through the fibre 7 to measuring cell 8. Probe 6 has a control sample making it possible to take account of the turbidity of the analysed solutions, i.e. all factors outside the material to be analysed which are liable to influence the absorption coefficient of the investigated solution. The light of intensity I'o leaving the optical reference probe 6 is reflected by mirrors 5 at exit 9 towards the not shown detector. The light of intensity I leaving the measuring cell 8 is reflected across the optical fibre 10 and mirrors 5 to the exit 11 and toward the same detector.
The measuring cell 8 is e.g. located in a protective enclosure 12 protecting it from the outside. This measuring cell 8 belongs to the control device of an industrial process, whose evolution is to be permanently monitored. The comparison of the intensities Io, I'o and I makes it possible to have information on the light absorption in measuring cell 8 due solely to the presence of the material to be analysed and consequently makes it possible to dose said material.
A known device like that shown in FIG. 1 suffers from a certain number of specific disadvantages, which will now be investigated. For this purpose, firstly certain general details will be given on the characteristics of optical fibres.
The transmission of an optical fibre varies with the wavelength and is a function of its composition (glass, plastic, silica with an index gradient or jump). The best transmission performances are presently obtained with synthetic silica fibres, the attenuation measured at 0.85.mu. wavelength being approximately 1 decibel/km (dB/km).
By choosing optical fibres with predetermined characteristics, it is consequently possible to envisage carrying out photometric measurements at a distance of several hundred meters, but without eliminating the considerable light energy loss resulting from a device like that of FIG. 1.
Thus, apart from the transmission loss due to the actual fibre, there are certain light energy loss causes at the junctions of the fibres, (particularly in collimating lenses and also during reflections at the intake of the fibres).
Thus, on adding the losses due to the addition of fibres (10 dB) to the connectors and the partition passages (10 dB), and finally to the measuring cell (5 dB), the attenuation provided by a coupling device of a photometer by optical fibres can reach 25 dB. Converted into absorption units, this 25 dB loss represents an optical absorption of 2.5 absorptivity losses, whereas conventional commercial spectrophotometers permit measurements in the range 2 to 4 absorptivity units, i.e. with a beam intensity of 10.sup.-2 to 10.sup.-4 of that of incident light. This consequently leads to a limitation to uses, because it is necessary to deduct from the scale dynamics of the apparatus, the loss resulting from the coupling device. This disadvantage is particularly prejudicial in the case of a device like that of FIG. 1, whereof the only light energy source is the internal source of the spectrophotometer, initially designed for a measurement in the sample compartment S of the apparatus.
Another problem linked with in-line industrial control must also be mentioned. A device like that of FIG. 1 only makes it possible to monitor a single point in an installation with the aid of spectrophotometer 1. However, in industry there are numerous cases where the production process of a product requires a number of controls at several successive points during the production of this product and in this case, on transposing the apparatus of FIG. 1 to a construction of this type, it is necessary to have the same number of spectrophotometers as there are measuring cells. Although such an equipment would be theoretically possible, it would be very heavy and costly and it is obviously preferable, for economic reasons alone, to have only a single spectrophotometric analyser able to exploit the different measuring stations.
Finally, the last problem resulting from the in-line industrial control usage of these spectrophotometers, which are not designed for this purpose, is that of the life of the light source or sources incorporated into the apparatus. These sources which emit, either in the ultraviolet (deuterium source) or in the visible and near infrared (quartz-iodine source) have variable lives ranging from 50 to 2000 hours as a function of their characteristics.
Such a life is not a major disadvantage in the conventional use in a laboratory, because when necessary it is merely a question of replacing the faulty lamp. However, this is not so under continuous control conditions in a factory, where the interruption of the measurement is sometimes not possible for safety reasons.